In a variety of medical diagnostic and therapeutic settings, as well as in biomedical research, it is desirable to image a subsurface target or area within a body region of a subject. For example, non-invasive locating and imaging of part or all of a solid tumor, an area of myocardial ischemia, the distribution of a therapeutic compound administered to the subject, or the progression of a disease may provide useful research or diagnostic information. Ideally, an imaging method is able to locate within a body region a target of interest, and provide information about the target's shape, size, number of cells, and depth below the surface of the body region. However, until now, methods that have been used and/or proposed for subsurface body target imaging have generally been limited to those using ionizing radiation such as X-rays, expensive and bulky equipment such as Magnetic Resonance imaging (MRI), or ultra-sound.
X-rays have excellent tissue penetration, and when used in conjunction with computed tomography (CT) or computed axial tomography (CAT), can produce superior image quality. However, X-rays have limited use in monitoring disease progress because exposure to X-rays is potentially harmful if such exposure is prolonged. X-rays can be used to locate and image compositions that have localized at a target within a body region, but always with exposure to the potential harm associated with X-ray radiation. X-rays, however, cannot readily be used to image the expression of gene products in vivo and determine the depth and/or shape of a target expressing such gene products.
MRI is also an excellent method for imaging targets, areas, and structures in a body region of a subject. Although MRI is not thought to possess harmful attributes like those associated with ionizing radiation, the expense and bulky equipment size needed to use MRI make it impractical for many applications or situations. MRI can provide two and three-dimensional information about targets within a body region of a subject, but is less effective at imaging physiological activity associated with a target.
Ultrasound or ultrasonography is the use of high-frequency sound (ultrasonic) waves to produce images of structures within the human body. Ultrasonic waves are sound waves that are above the range of sound audible to humans. Ultrasonic waves are produced by the electrical stimulation of a piezoelectric crystal and such waves can be aimed at specific body regions. As the waves travel through body tissues within a body region, they are reflected back at any point where there is a change in tissue density, as, for instance, in the border between two different organs of the body. Ultrasound offers the advantages of not using radiation or radioactive material, and employs lesser expensive and less bulky equipment than MRI, but is limited to only discerning differences in density of underlying tissue and structures. Accordingly, ultrasound cannot effectively track and monitor the progress of an infection unless such infection results in a discernable shift in density of the target tissue. Ultrasound cannot image or detect the physiological functions of tissues or organs.
Until now, Positron Emission Tomography or P.E.T. was unique among imaging techniques because it produces an image of organ or tissue function. Other imaging techniques such as X-ray, CT, MRI, and sonography depict organ or tissue anatomy but cannot discern physiological activity within them. To image a specific biochemical activity of an organ, a radioactive substance, called a radiotracer or radiopharmaceutical, is injected into the body or inhaled. The tracer is usually a radioactive equivalent of a substance that occurs naturally within the body such as water or sugar. The radioactive isotope is identical to the body's own nonradioactive isotope except that its atoms have a different number of neutrons. Consequently, a subjects body is burdened with radioactive material, and the potential harm associated with such material. P.E.T cannot detect non-isotopic expression products from transgenic tissues, organs, or transgenic organisms. Scintigraphy, a diagnostic technique in which a two-dimensional picture of a bodily radiation source is obtained by the use of radioisotopes, may also be used for imaging structures and their functions. Scintigraphy, however, is not suitable for determining the depth of a target in a body region of a subject.
In view of the above-mentioned technologies for locating and imaging a target in a body region of a subject, there is a need for methods and devices to determine the depth and/or the shape and/or number of cells of such target without having to use radioactivity, radiation, or expensive and bulky equipment. The invention disclosed herein meets these needs.